systemd: Do not install anything in /var/volatile

/var/log is typically a symbolic link to inside /var/volatile, which
is expected to be empty. Check ${VOLATILE_LOG_DIR} to see if it is ok
to install /var/log.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>